The longer answer is "why not?" The paid membership gives you access to a boat load of materials for professional project management. I get it that professional society memberships are expensive: I pay well over $1000 USD per year to maintain society memberships because there is value in those memberships. I can categorically state that ROI for PMI membership is higher than the other two professional societies of which I belong.
Training for Project Management Professional (PMP)®, PMI Agile Certified Practitioner (PMI-ACP)®, and Certified Associate in Project Management (CAPM)®